A cigar in transit passes through temperatures and humidities nobody chose for it, and arrives either drier or wetter than it left. Neither state is evenly distributed: the surface responds to the outside air far faster than the core, so a cigar can read correct on the wrapper while its filler has not caught up.
Is resting the same as ageing?
No, and conflating them is the common error. Resting is equalisation and it is measured in weeks. Ageing is slow chemical change in the leaf, measured in years, and it is a different question with different answers. A cigar that has rested is ready; a cigar that has aged is altered.
How long is long enough?
Two weeks to a month covers most shipped cigars. One that arrived visibly dry needs longer, and needs it brought back slowly — a fast rehydration expands the filler against a wrapper that has not softened at the same rate, which is how a wrapper splits.
What actually changes
Moisture redistributes from the surface inward or outward until the whole cigar is at one figure. That is why the fix for a badly behaved cigar is very often time rather than technique.